The present invention relates to a device for admitting a laser beam into an end of an optical fiber.
Unlike normal or usual light, laser beam is uniform in phase and is superior in convergence and, therefore, it is possible for the laser beam to concentrate beam energy of extremely high density on a narrow area. Accordingly, the laser beam is applicable to machinings, medical treatments and the like. The laser beam may be supplied by a laser beam generator through an optical fiber. In this case, the laser beam from the generator is admitted into one end of the optical fiber through a condensing optical system and is irradiated from the other end of the optical fiber.
FIGS. 1 and 2 illustrate a laser beam admitting device of the prior art, which comprises a metallic support structure 50. A support bore 51 is formed in the support structure 50 so as to extend along a longitudinal axis thereof, and is comprised of a large- diameter bore section 51a and a small-diameter bore section 51b located at an axial forward end of the bore section 51a.
The support structure 50 is adapted to support an optical fiber 10 so as to locate a beam admitting end 10a of the optical fiber 10 at a focus position of a convex lens 20 which serves as a condensing optical system. In FIG. 1, the optical fiber 10 is illustrated as having its diameter exaggerated as compared with an actual one.
The optical fiber 10 is comprised of a core 11 and a clad layer 12 surrounding the core 11. In addition, the optical fiber 10 is covered with an envelope 14 through a resilient support layer 13. The optical fiber 10 has its forward end portion from which the resilient support layer 13 and the envelope 14 are removed so that the clad layer 12 is exposed.
The envelope 14 of the optical fiber 10 is inserted into and supported by the large-diameter bore section 51a in the support structure 50, and the exposed clad layer 12 of the optical fiber 10 is inserted into and supported by the small-diameter bore section 51b.
With the construction as described above, a laser beam of high energy condensed by the convex lens 20 is admitted from the beam admitting end 10a of the optical fiber 10 into the core 11 thereof, passes through the core 11, and is irradiated from the other end of the optical fiber 10.
When the optical fiber 10 is supported by the support structure 50 constructed as described above, various problems are encountered. Specifically, since the core 11 of the optical fiber 10 is extremely fine in diameter, it is difficult to condense all of the laser beam onto the narrow area of the core end, and there may be a case where a part of the laser beam is admitted into the clad layer 12, as shown in FIG. 2. The laser beam admitted into the clad layer 12 passes through the core 11 while being refracted, and is again admitted into the clad layer 12 while being further refracted. In a portion of the clad layer 12 projecting from a forward end face of the support structure 50, the laser beam is totally reflected from an outer circumferential surface of the clad layer portion, and is transmitted toward the other end of the optical fiber 10 while repeating the aforesaid refraction. At a location where the clad layer 12 is surrounded by the small-diameter bore section 51b in the support structure 50, however, the laser beam reaching an inner peripheral surface of the small-diameter bore section 51b is absorbed thereat and, as a result, the beam energy is converted into heat. Since the inner peripheral surface of the small-diameter bore section 51b is relatively small in surface area and an amount of heat generated per unit area is great, problems occur that the support structure 50 is locally heated to a high temperature; the forward end portion of the optical fiber 10 is melted down and damaged; and the like.
In order to avoid the aforementioned problems, it may be considered as a possibility to take such a measure as to transmit the laser beam admitted into the clad layer 12, through the optical fiber 10, without conversion of the admitted laser beam to heat in the vicinity of the beam admitting end 10a of the optical fiber 10. In this case, however, it will be presumed that some problems arise. That is, if the optical fiber 10 is required to be bent at an acute angle, the laser beam admitted into the clad layer 12 would leak from the bent portion so that the envelope 14 at the bent portion is heated and softened. By this softening, the bent portion would tend to be further bent, and such a vicious circle would be caused that the further bending promotes the leakage of the laser beam. As a result, a mechanical breakage due to stress concentration or a melt-down due to heat might occur.